#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

#define NMAX 100002

double X[NMAX],Y[NMAX];
int n;

int main()
{
    f>>n;
    for(int i=0;i<n;i++)
    {
        f>>X[i]>>Y[i];
    }
    X[n]=X[0];
    Y[n]=Y[0];
    double s=0;

    for(int i=0;i<=n;i++)
    {
        s+= X[i]*Y[i+1] - X[i+1]*Y[i];
    }
    g<<s/((double)(2))<<"\n";
    return 0;
}